Telephones and other types of data terminal equipment (DTE) are routinely used for voice, data traffic and other forms of telecommunication. Such DTE equipment typically is wired with twisted pair wire to a switch or similar telecommunications device. For example, some communications systems utilize an Ethernet switch in communication with Internet Protocol (IP) or voice over IP (VoIP) telephones. Where the IP telephones are compatible and thus adapted to receive phantom power over the twisted pair connection to the switch, it is desirable for the switch to verify the compatibility before applying the phantom power because it is conceivable that the phantom power could damage or operate improperly with certain non-compatible DTE equipment (“legacy equipment”) which might also be connected to the switch. In accordance with the invention disclosed in co-pending U.S. patent application Ser. No. 09/710,388 filed Nov. 9, 2000 in the name of inventor Roger Karam and entitled “Method and Apparatus for Detecting a Compatible Phantom Powered Device Using Common Mode Signaling”, commonly owned herewith, a method and apparatus which enable discovery of such compatible telephones by a switch or similar device is taught. This application issued on Oct. 12, 2004 as U.S. Pat. No. 6,804,351. In a nutshell, the approach used is to generate a differential mode signal, apply it to center-taps of transformers coupling the switch to the twisted pair wires, apply the differential mode signal received at center-taps of corresponding transformers at the IP telephone to an identity network, loop the signal (possibly modified by the identity network) back to the switch, and, based on the returned signal (and possibly other considerations), apply or not apply phantom power between the center-taps of the switch-side transformers to power the IP telephone. This approach requires that the IP telephone be configured to “loop back” signals received by it to the switch. This is undesirable for data signals under certain circumstances as it can lead to certain kinds of potential computer network problems. Accordingly, it is desirable in such circumstances to permit loop back of discovery signals only and not data signals. In the past, normally closed mechanical relays at the IP telephone coupled with a low pass filter (LPF) to pass only the discovery signals and not the data signals have been used. Such mechanical relays are relatively expensive and can become unreliable. Low pass filters composed of inductors and capacitors also consume volume in the DTE equipment and can be relatively expensive to deploy.
FIG. 1 is an electrical schematic diagram of a telecommunications system in accordance with a prior design. A telecommunications device 10 such as an Ethernet switch includes a port 12 which includes a transmitter 14 and a receiver 16. Transmitter 14 includes a center-tapped transformer winding 18 with differential output on nodes 20, 22 and a center-tap 24. Receiver 16 includes a center-tapped transformer winding 26 with differential input on nodes 28, 30 and a center-tap 32. A phantom power supply 34 provides direct current (DC) phantom power (preferably=+48 volts or less) to center taps 24, 32. A four (or more) wire cable 36 connects telecommunications device 10 to, for example, an IP telephone 38. IP telephone 38 receives a differential signal at nodes 40, 42 of receive transformer 44 which includes center-tapped winding 46. IP telephone 38 transmits a differential signal at nodes 48, 50 of transmit transformer 52 which includes center-tapped winding 54. Center-tap node 56 is the center-tap of winding 46 and center-tap node 58 is the center-tap of winding 54. Phantom power is extracted at nodes 56, 58 and is applied to a power processor 60 at the IP telephone in known ways, such as is taught in U.S. Pat. No. 6,115,468 filed Mar. 26, 1998 entitled “Power Feed For Ethernet Telephones Via Ethernet Link” and commonly owned herewith. A first relay 62 couples differential output lines 64, 66 when unenergized to low pass filter network 68. A second relay 70 couples the differential outputs 72, 74 of LPF 68 to differential input lines 76, 78 of winding 80 of transformer 52. In this way, while relays 62, 70 are not energized (as is the case when phantom power is not applied), signals loop through IP telephone 38 but they are subjected to LPF 68 which filters out the higher frequency data signals while allowing the lower frequency discovery signals to pass.
When relays 62, 70 are energized (e.g., when phantom power supply 34 for port 12 is turned on or another condition controlling relays 62, 70 is met) then the receive signals from differential output lines 64, 66 of winding 82 of transformer 44 are directly applied to the physical layer device (PHY) 84 of IP telephone 38. Similarly this condition causes transmit signals from PHY 84 to be coupled to output winding 80 of transformer 52.
The details of a common low pass filter 68 are shown by way of example in FIG. 2. FIG. 2 is a typical LPF circuit including three capacitors C1, C2, C3, and four inductors L1, L2, L3, L4. The input signal is differential and is applied at modes IN+, IN− and the output signal is differential and is obtained at modes OUT+, OUT−. Such devices are difficult to integrate onto an integrated circuit with current technology and thus must actually be fabricated with discreet components or is known to those of ordinary skill in the art.
Relays 62 and 70 and LPF 68 are physically relatively large and tend to be relatively expensive parts. Furthermore, relays can wear out and/or suffer from intermittent failures and are thus not considered to be the most reliable of electronic devices. Accordingly, it is desirable to replace the need for relays and discreet filter components in circuits of this type and to further miniaturize the loop back control circuit.